Bone engineering with osteoprogenitor cells has enormous clinical potential for the treatment of aging-related bone loss (osteoporosis) or traumatic bone defects. Osteoprogenitor cells such as mesenchymal stem cells (MSCs) from bone marrow are a source of such cells. When combined with polymeric scaffolds and/or osteogenic growth factors these cells may provide new therapies for bone replacement. Our long-range objective is the development of new treatments for human bone loss through tissue engineering. Our central hypothesis is that human adipose tissue contains stem cells or adipose-derived stem cells (ADSCs) which offer advantages over MSCs and other osteoprogenitor cell types. The specific aims of this application are: (1) to clone and characterize human ADSCs, (2) to investigate their ability to form bone both in vitro and in vivo and (3) to determine their ability to repair non-healing bone defects. An understanding of ADSC function and ultimately their regulation, has important implications. First, tissue engineering strategies will benefit by an autologous stem cell source (adipose tissue) that is easily obtainable in 'liter' quantities through a minor surgical procedure (liposuction) that is well tolerated by patients. Second, a detailed understanding of the regulation of stem cell differentiation in adipose tissue could significantly impact the treatment of diseases that are characterized by dysregulated mesodermal cell growth and differentiation such as osteoporosis, heterotopic calcification and obesity. Finally, fundamental issues of mesodermal cell differentiation, mesodermal phylogeny and ontogeny, may be better understood by study of these cells. At the completion of this grant, our expectation is that human adipose tissue will be shown to be a reservoir of stem cells. We will also begin to have a basic understanding of the phenotypic changes occurring in differentiating ADSCs after commitment to the osteogenic lineage. Finally, we will assess the clinical utility of ADSCs to repair critical-sized bone defects.